1. Field of the Invention
The present invention concerns cellular mobile radio systems.
The present invention is applicable to cellular mobile radio systems including cells of low capacity (for example microcells), for example.
Accordingly, in a time division multiple access system such as the GSM system, for example, the present invention applies to cells with a capacity limited to eight channels, i.e. seven dedicated channels plus one common channel, for example.
2. Description of the Prior Art
The architecture of a fixed station, or base transceiver station, for a cell of this kind in the GSM system is shown in FIG. 1 and the principle of sending by a base transceiver station of this kind is shown in FIG. 2.
In the GSM system the time division multiple access temporal structure is made up of repetitive frames each including eight time slots IT.sub.0 through IT.sub.7.
In addition to traffic data relating to said dedicated channels, a base transceiver station in the GSM system must send control data relating to said common channel in a particular time slot (in this instance the first time slot IT.sub.0) of a frame structure transmitted on a frequency called the BCCH frequency.
The base transceiver station shown in FIG. 1 therefore includes a transmitter E.sub.0 sending a frequency f.sub.0 (constituting the BCCH frequency) receiving the data to be sent in the various time slots of said time structure, this data being originated by baseband processing means T (encoding and conversion to the GSM system transmission format).
This data includes, in this instance:
data forming a common channel CH.sub.0 conveyed by the time slot IT.sub.0, as shown in FIG. 2, PA1 data forming dedicated channels CH.sub.1 through CH.sub.7 respectively conveyed by the time slots IT.sub.1 through IT.sub.7, as shown in FIG. 2. PA1 different base transceiver stations receive the same time reference signal from a switching center over different wired links, PA1 one of these base transceiver stations is a reference base transceiver station, the others being subordinate base transceiver stations, PA1 each subordinate base transceiver station listens to a mobile station communicating with the reference base transceiver station, PA1 each subordinate base transceiver station detects a time shift between the time reference signal as received from the switching center and the signal received from the mobile station, and applies a correction to compensate this time shift and therefore to synchronize to the reference base transceiver station. PA1 means enabling a mobile station to listen to said reference base transceiver station and said subordinate base transceiver station, PA1 means for deducing the time shift D.sub.1 between said two base transceiver stations as seen from said mobile station, PA1 means for determining the propagation time difference D.sub.2 between the mobile station and each of said base transceiver stations on the occasion of a handover involving said reference base transceiver station and said subordinate base transceiver station, and PA1 means for determining, from the values D.sub.1 and D.sub.2, a time shift to be corrected in order to synchronize said subordinate base transceiver stations to said reference transceiver station.
Mobile radio systems use the technique known as frequency hopping which improves transmission quality in the presence of the phenomenon of fading, for example.
This technique requires a minimum set of spectral resources, however; for example, it is not applicable to the base transceiver stations of the type shown in FIG. 1, since these base transceiver stations have only one carrier frequency.
Document WO 91/13502 discloses pooling frequencies allocated to different cells of the same re-use pattern of a cellular system for frequency hopping so that the frequency used during a time slot in one cell of the re-use pattern is different from those used in the same time slot in the other cells of the re-use pattern.
In the above document, it is assumed that no overlapping occurs that could produce interference.
However, this application proceeds from the position that this assumption cannot be made, i.e. that the various base transceiver stations of the system cannot be perfectly synchronized, but nevertheless aims to avoid any such risk of interference.